As is known, magnetoresistive sensors exploit the capacity of appropriate ferromagnetic materials (called magnetoresistive materials, for example the material known by the name “permalloy”, formed by an Fe—Ni alloy) to modify their resistance in the presence of an external magnetic field.
Currently, magnetoresistive sensors are obtained from magnetoresistive material strips. During manufacture, the magnetoresistive material strip can be subjected to an external magnetic field so as to have a preferential magnetization in a preset direction (referred to as the easy axis), for example the longitudinal direction of the strip.
Before measuring the external magnetic field, a state of initial magnetization along the axis of preferential magnetization is imposed via a current pulse through a set/reset strap. In absence of external magnetic fields, the magnetization maintains the direction imposed by the set/reset pulse, and the strip has maximum resistance in this direction. In presence of external magnetic fields having a direction different from that of preferential magnetization, the magnetization of the strip changes, as does its resistance, as explained hereinafter with reference to FIG. 1.
In FIG. 1, a magnetoresistor 1 is formed by a magnetoresistive material strip having a longitudinal direction parallel to the axis X, which forms also the direction of preferential magnetization. The magnetoresistor 1 is traversed by a current I flowing in the longitudinal direction of the strip. An external magnetic field Hy is directed in a parallel direction to the axis Y and causes a rotation of the magnetization M through an angle α with respect to the current I. In this case we haveR=Rmin+Rd cos2αwhere Rmin is the resistance of the magnetoresistor in case of magnetization M parallel to the axis Y (very high external magnetic field Hy), and Rd is the difference of resistance Rmax−Rmin, where Rmax is the resistance in case of magnetization directed in a parallel direction to the direction X.
For permalloy, the maximum ratio Rd/R is in the region of 2-3%.
Setting
                    sin        2            ⁢      α        =                                        Hy            2                                Ho            2                          ⁢                                  ⁢        for        ⁢                                  ⁢        Hy            ≤      Ho        and                    sin        2            ⁢      α        =                  1        ⁢                                  ⁢        for        ⁢                                  ⁢        Hy            ≥      Ho      where Ho is a parameter depending upon the material and the geometry of the strip 1, we have:
                    R        =                                            R              min                        +                                                            R                  d                                ⁡                                  [                                      1                    -                                                                  (                                                  Hy                          Ho                                                )                                            2                                                        ]                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              Hy                                ≤          Ho                                    (        1        )            
FIG. 2 represents with a dashed line the plot of the resistance R resulting from Eq. (1) (curve A).
It is moreover known, in order to linearize the plot of the resistance R at least in an operative portion of the curve, to form, above the magnetoresistive material strip, transversal strips 2 (called “barber poles”), of conductive material (for example aluminum), set at a constant distance and with inclination of 45° with respect to the direction X, as shown in FIG. 3.
In this situation, the direction of the current I changes, but not the magnetization. Consequently, Eq. (1) becomes:
                    R        =                                            R              min                        +                                                            R                  d                                2                            ±                                                                    R                    d                                    2                                ⁢                                  (                                      Hy                    Ho                                    )                                ⁢                                                      1                    -                                                                  (                                                  Hy                          Ho                                                )                                            2                                                                      ⁢                                                                  ⁢                for                ⁢                                                                  ⁢                Hy                                              ≤          Ho                                    (        2        )            having a linear characteristic around the point Hy/Ho=0, as shown by the curve B, represented by a solid line in FIG. 2.
In practice, in this neighborhood, the term under the square root is negligible as compared to the linear term and thus we have
                    R        =                              R            o                    ±                      k            ⁡                          (                              Hy                Ho                            )                                                          (        3        )            
The ± sign in Eq. (3) depends upon the direction of the transversal strips 2 (±45°).
FIG. 4 shows a magnetoresistive sensor 9 including four magnetoresistors 1 having transversal strips 2 arranged in an alternating way. The magnetoresistors 1 are connected so as to form a Wheatstone bridge formed by two mutually parallel branches 3, 4 defining input terminals 5, 6 and output terminals 7, 8. In detail, in each branch 3, 4, the two magnetoresistors 1a, 1b have transversal strips 2 directed in an opposite direction (+45° and −45°, respectively). The magnetoresistors 1a, 1b of one branch are arranged diametrically opposite to the corresponding ones of the other branches (the magnetoresistor 1a of the first branch 3 with transversal strips 2 at +45° is connected to the second input terminal 6 and the magnetoresistor 1a of the second branch 4 with transversal strips 2 at +45° is connected to the first input terminal 5; the same applies to the magnetoresistors 1b). A biasing voltage Vb is applied between the input terminals 5, 6.
Trimmer resistors can be connected in series to each branch 3, 4, in a way not shown, so that, in absence of an external magnetic field directed in a parallel direction to the direction of detection (here the field Hx), the output voltage Vo across the output terminals 7, 8 is zero. Instead, in case of initial magnetization directed vertically downwards, an external magnetic field Hx causes an increase in the resistivity of the magnetoresistors, here the straps 1a, having transversal strips 2 directed at +45° and a corresponding reduction in the resistivity of the other magnetoresistors 1b having transversal strips 2 directed at −45°. Consequently, each variation of resistance due to an external field perpendicular to the magnetoresistors 1a, 1b causes a corresponding linear variation of the output voltage Vo, the value of which thus depends in a linear way upon the external magnetic field Hx.
Because of the high sensitivity of magnetoresistive sensors of the type referred to above, recently use thereof has been proposed for electronic compasses in navigation systems. In this case, the external field to be detected is represented by the Earth's magnetic field. To a first approximation, the Earth's magnetic field can be considered parallel to the Earth's surface and the reading of the compass thus requires two sensors sensitive to the two directions of the plane locally tangential to the Earth's surface.
Since, however, the inclination of the compass with respect to the tangential plane entails reading errors, to correct these errors three sensors are used, each having a sensitive axis directed according to the three spatial axes X, Y, Z.
To this end, the three sensors are arranged with their sensitive axes positioned 90° with respect to each other. Whereas the production of a sensor sensitive to fields directed in two directions does not create any difficulty, since they lie in the same plane, having the third sensor in the third direction involves a plane perpendicular to that of the first two sensors, as shown in FIG. 5, where the sensors X and Y are integrated in a chip 10 and the sensor Z is integrated in a different chip 11 and the chips 10, 11 are fixed to a same base or frame 12. In fact, in this case, the operations of assembly are much more complex and the finished device is much more costly.
In addition, the alignment tolerances between the sensor Z and the sensors X and Y provided in different chips are greater than in case of sensors integrated in a single chip so that a smaller precision is achieved as regards determination of the direction of the magnetic field, which is fundamental for the applications of an electronic compass.
In addition, with the scaling down of the chips, the packages should be increasingly small (e.g., from 5×5 mm2 to 3×3 mm2); however, vertical assemblage is incompatible with the desired reduction.
The solutions proposed to the problem indicated are not, however, satisfactory. For example, United States Patent Application Publication No. 2009/0027048 (incorporated by reference) describes a manufacturing process wherein a magnetoresistance is deposited in a V-shaped trench so that the sensitive layer is able to detect also part of the component perpendicular to the chip. On the other hand, this solution renders more difficult deposition and definition of the transversal strips or “barber poles”, of the metal interconnections, and of the auxiliary straps for calibration and for the set-reset procedure (the so-called “flipping”) for reduction of offset.
Similar problems exist also in case of a single sensor for detecting magnetic fields directed perpendicularly to the horizontal plane, when the vertical arrangement of the device including the sensor is not possible or when, even though the aim is to detect the horizontal field components, it is necessary to arrange the device in a vertical position.